Nitride semiconductor devices typified by HFETs (heterostructure field effect transistors) formed of a nitride semiconductor are used as semiconductor devices for electric power used for switching power sources, inverter circuits, etc., and are expected to have a higher breakdown voltage and lower ON resistance than semiconductor devices for electric power formed of silicon. When an inductance is included among the loads, it is necessary to allow the current flowing through the inductance to continue to flow as a circulating current when the nitride semiconductor device has become OFF. Hence, the nitride semiconductor device needs to be connected in antiparallel to a freewheel diode. To downsize switching power sources and inverter circuits, it is desired for the nitride semiconductor device to include a freewheel diode connected in antiparallel. A normally off type is desired for the nitride semiconductor device, but in a normally off nitride semiconductor device, resistance is high in a portion immediately below a gate electrode of an electron transit layer through which an operating current flows. For the normally off nitride semiconductor device including a freewheel diode connected in antiparallel, it is desired to have a high breakdown voltage, low ON resistance, and low forward voltage of the freewheel diode.